测绘工程专业英语

MethodsofElevationDetermination

Anelevationisaverticaldistanceaboveorbelowa

ghverticaldistancecanbereferenced

toanydatum,insurveying,thereferencedatumthatis

universallyemployedisthatofmeanalevel(MSL).MSLis

assignedaverticalvalue(elevation)of0.000ftor0.000m.

Allotherpointsontheearthcanbedescribedbytheelevations

aboveorbelowzero.

Permanentpointswhoelevationshavebeenprecily

determined(benchmarks)areavailableinmostareasforsurvey

a,7yearsofobrvationsattidalstationsin

Qingdaofrom1950to1956werereducedandadjustedtoprovide

1987,thisdatum

wasfurtherrefinedtoreflectlongperiodicaloceantide

changetoprovideanewnationalverticaldatumof1985,

accordingtotheobrvationsattidalstationsfrom1952to

gh,strictlyspeaking,thenationalverticaldatum

maynotprecilyagreewiththeMSLatspecificpointsonthe

earth’ssurface,thetermMSLisgenerallyudtodescribe

ssignedaverticalvalue(elevation)of0.000

enceinelevationmaybemeasuredbythe

followingmethods:

orspiritleveling,bymeasuringverticaldistances

levelingismostprecimethodofdetermining

elevationsandtheonecommonlyud.

ctortrigonometricleveling,bymeasuringvertical

anglesandhorizontalorslopedistances.

leveling,inwhichverticaldistancesaredetermined

bytacheometryusingengineer’stransitandlevelrod;

plane-tableandalidadeandlevelrod;orlf-reducing

tacheometerandlevelrod.

tricleveling,bymeasuringthedifferencesin

atmosphericpressureatvariousstationsbymeansofa

barometer.

etricleveling,bymeasuringthedifferencesin

gravityatvariousstationsbymeansofagravimeterfor

geodeticpurpos.

alpositioningsystem,inwhichaninertialplatform

hastreemutuallyperpendicularaxes,oneofwhichis“up”,

sothatthesystemyieldlevationasoneofthe

alaccuraciesfrom15to50cmindistancesof

60and100km,respectively,ipment

costixtremelyhighandapplicationsarerestrictedtovery

largeprojectswhereterrain,weather,time,andaccessimpo

specialconstraintsontraditionalmethods.

veyelevationsarereferencedtotheellipsoidbut

canbecorrectedtothedatumifasufficientnumberofpoints

withdatumelevationsarelocatedintheregionsurveyed.

Standarddeviationsinelevationdifferencesof0.053to0.094

marepossibleundertheconditions.

Spiritleveling

Themostprecimethodofdeterminingelevationsandmost

commonlyumethodisdirectlevelingorspiritlevelingwhich

ential

levelingisudtodeterminedifferencesinelevationbetween

pointsthatareremotefromeachotherbyusingasurveyor’

mple,

todeterminetheelevationsofdesiredpointBwithrespectto

apointofknownelevationA(eFigure1),theelevationof

which(BM)isknowntobeabovealevel,thelevelistup

atintermediatepointbetweenAandB,androdreadingsare

e

elevationofthelineofsightoftheinstrument(being

horizontal)isknowntobethelineofsightoftheinstrument

HA+vationofpointBcanbedeterminedbyequation

HB=HA+a－b

InadditiontodeterminingtheelevationofpointB,the

elevationsofanyotherpoints,lowerthanthelineofsight

andvisiblefromthelevel,canbedeterminedinasimilar

calledBacksight(BS)whichisarodreadingtakenonapoint

ofknownelevationinordertoestablishtheelevationofthe

ledForesight(FS)whichis

arodreadingtakenonaturningpoint,benchmark,ortemporary

+arefers

totheHeightofInstrument(HI)whichistheelevationofthe

orefraction,actually

thelineofsightisslightlycurved,theeffectsofcurvature

andrefractionforthehorizontaldistancecanbereducedto

anegligibleamountandnocorrectionforcurvatureand

refractionisnecessaryifbacksightandforesightdistances

arebalancedinpracticaloperation.

TrigonometricLeveling

Trigonometriclevelingisudwheredifficultterrain,such

asmountainousareas,precludestheuofconventional

ernapproachistomeasurethe

slopedistanceandverticalangletothepointinquestion.

Slopedistanceismeasuredusingelectromagneticdistance

measurersandthevertical(orzenith)angleusingatheodolite,

orthetotalstationthatintegratethetwoinstrumentsinto

tationscontainbuilt-in

microprocessorsthatcalculateanddisplaythehorizontal

distancefromthemeasuredslopedistanceandverticalheight.

Thislatterfacilityhasresultedintrigonometricalleveling

beingudforawidevarietyofheightingprocedures,

icconceptoftrigonometrical

asuringthevertical

angleαandthehorizontaldistanceSisud,thenthe

differenceinelevationhABbetweengroundpointsAandBis

therefore:

hAB=S×tanα+i–v

whereiistheverticalheightofthemeasuringcenterofthe

instrumentaboveAandvistheverticalheightofthecenter

ticalanglesarepositivefor

zenithanglesarealwayspositive,butnaturallywhengreater

than90°ometrical

levelingmethodofdeterminingdifferenceinelevationis

limitedtohorizontaldistancelessthan300mwhenmoderate

precisionissufficient,andtoproportionatelyshorter

distancebeyond

300mtheeffectsofcurvatureandrefractionmustbeconsidered

inatetheuncertaintyinthecurvatureand

refractioncorrection,vertical-angleobrvationsaremadeat

bothendsofthelineascloinpointoftimeaspossible.

Thispairofobrvationsistermedreciprocalvertical-angle

rectdifferenceinelevationbetweenthe

twoendsofthelineisthemeanofthetwovaluescomputedboth

wayitherwithorwithouttakingintoaccountcurvatureand

ortantnotesshouldbementionedhereisthat

surveyorsudtoworkingwithspiritlevelshavereferenced

orthometricheights(H)tothe“average”surfaceoftheearth,

r,theelevationcoordinate(h)given

byGPSsolutionsreferstotheheightfromthesurfaceofthe

ellipsoidtothegroundstation.